Current aircraft enhanced visual systems (EVS) are designed to provide improved orientation during day and night taxiing or flying, and to enable visual landing in bad atmospheric conditions such as fog, haze, dust, or smog. For example, such system may display a captured video image superimposed on the pilot field of view. During a regular landing approach, the plane descends at an angle of 3 degrees. At the decision height (DH), which is approximately 200 feet above ground, the pilot has to recognize the structure of the lights in the runway. If, at the DH, the pilot is not able to recognize the runway light structure, he has to make a missed approach. Assuming a descending angle of 3 degrees and a decision height of 200 feet, the distance between the plane to the lights structure is about 1165 meters.
The atmospheric transmittance for landing applications is defined by a value called the runway visibility range (RVR). One (1.0) RVR is the maximum distance for runway light detection by the naked eye. The mathematical meaning of this definition is that RVR is defined as the distance at which the light intensity drops to 2% contrast from the contrast value at zero distance. The contrast value at zero distance is one (1.0). For each additional RVR distance, the runway bulb signal decreases by a factor of 50. Current systems have a maximal penetration distance of 2.2*RVR. The atmospheric attenuation, in the visible spectral band and in the near IR, for a distance of 2.2 RVRs, is equal to 502.2, which is approximately 5500. In order to penetrate such a large atmospheric attenuation, current systems provide a signal-to-noise ratio (SNR) of about 5500 at zero distance. In other words, the EVS camera should provide a signal-to-noise ratio of 5500 in laboratory conditions. In order to obtain the required SNR, the minimum number of photoelectrons to be collected per pixel per frame is 30,000,000 (or 3×107). However, the quantum noise is not the only source of noise. In order to overcome the additional noise sources such as the readout noise, the fixed pattern noise, etc., the minimum number of photoelectrons to be collected per pixel per frame have to exceed 30*106. The SNR requirement is a necessary but not sufficient condition.
In most airports, the runway lights are mounted on short poles above ground level. The bulbs are exposed to wind and rain; therefore the bulbs' temperature difference against the background is very low. The bulb's exterior material is glass. Glass is opaque above 2.2 microns. The conclusion is that in bad atmospheric conditions, the only spectra available for bulb detection are in the visible or near IR spectral bands. Current systems use spectral band ranges between 1.2 and 5.0 microns that are within the transmittance range of silicon and are optimal for the InSb focal plane array detectors.